1. Field of the Invention
This invention relates to image recording processes and more particularly to employing self-correcting processes of forming images in connection with photolithography of multiple images aligned to each other or a like process akin to printing onto a previously imaged substrate.
2. Description of Related Art
FIGS. 1-3 show a perfect process flow, in the prior art, for manufacture of a device 10. The intent here is to sequentially produce two images leading to formation of two openings in semiconductor layers. Typically a spatial relationship between the edges of two openings in overlaying imaging levels is to be reproduced to tight manufacturing tolerances.
Referring to FIG. 1, in the practice of prior art a device 10 is formed on a semiconductor substrate 11, which is shown with a photoresist etch mask 12 formed thereon with an opening 6 therethrough which was used to form a hollow opening B with sidewalls 7 in the substrate 11 by anisotropic etching.
Referring to FIG. 2, the device 10 of FIG. 1 is shown after the mask 12 has been removed from substrate 11 and a film 13, e.g. an oxide layer, has been formed by any method desired (that narrows down the width of opening B) to form a narrower opening 6' with sidewalls 16 centered on opening B. This image modification is achieved strictly by process means. As a result, this derivative image defined by sidewalls 16 of opening 6' is self-aligned to the opening B with sidewalls 7 and it maintains the same center as opening B, as well as replicating the shape of B in an inward direction while retaining excellent centerline level-to-level overlay component of edge-to-edge overlay. Then a new blanket photoresist layer 14 has been formed on the device 10 covering dielectric film 13.
Then a new photolithographic image of opening 9 in an optical mask (or reticle) 8 is formed by exposing photo-resist layer 14 of FIG. 2 and developed into a well opening 15 in photoresist mask 14' as seen in FIG. 3. The photo-resist layer 14 of FIG. 2 has been formed into a photoresist etch mask 14' of FIG. 3 with the pattern for an opening 15 above the opening B. The photoresist etch mask 14' is used to etch opening E (with sidewalls 18) in the film 13 and, possibly, substrate 11 (as illustrated in FIG. 3). Level-to-level, edge-to-edge overlay in the plane of the top surface of the substrate 11 of the opening 15 (in resist etch mask layer 14') with the opening B limited by sidewalls 7 or with its derivative defined by sidewalls 16 is critical.
In order to maintain tight production control of this edge-to-edge overlay, dimensions of features in both masking levels and level-to-level centerline overlay must be simultaneously controlled. FIG. 3 illustrates perfect overlay of the openings E and B in the substrate 11, as desired in an ideal result.
FIG. 4 shows a typical problem with practicing the prior art approach of FIGS. 1-3. Specifically the centerline level-to-level overlay error is illustrated, where, at the time of exposure, the opening 9 in mask 8 is misplaced to the left from its intended position. The result is formation of opening 15' in photoresist layer 14', also misplaced to the left, and a subsequent etching of opening 18' (E) into the substrate 11 with encroachment on the left sidewall 16 of well B formed by film 13. In other words, because of misalignment of opening 9 in mask 8 with opening B in substrate 11, its image defined by sidewalls 15' in the photoresist 141 has been produced too far to the left. An error of making the width of opening 9 in mask 8 or of its image 15' in the photoresist 14' too large would lead to a similar problem.
When tight tolerances of level-to-level centerline overlay and linewidth (critical dimension, CD) for both openings B and E are not maintained, opening E may be defined outside of the region bounded by sidewalls 16" of film 13 within opening B, resulting in a device malfunction.
Making an opening E is a new patterning step which, in the present state of the art, does not take account of the existing (previously patterned on substrate) pattern B. At the most, measures are taken to suppress influence of the prior pattern on the new pattern formed, as in antireflection coatings, dyed photoresist, multilayer photoresist systems, surface imaging, etc., which are all expensive enhancements for the problem of opening (image) E size definition or tolerancing, each detrimentally affected by the existence of prior pattern B.
Using all of the above art does not overcome the underlying problem of proper image size and centerline overlay control, concurrently, in many imaging layers (which leads to device malfunction.) While these approaches mitigate the problem, they do it in an unsatisfactory way, because of the unacceptable levels of complexity and expense. The existing imperfections in masks(reticles) and printing machines (image distortion) and image size variation in the prior layer containing opening B and the new imaging layer (containing opening E) are not even addressed. The prior art is based upon the philosophy of control of all tolerances individually (for definition of typical current practices of prior art see Booth et al., "A Statistical Approach to Quality Control of Non-normal Lithographic Overlay Distortion", IBM J. Res. Develop., Vol. 36, pp. 835-844; 1992).
Maintaining the required lithographic tolerances is a difficult task which requires ever increasing complexity of microlithographic masks and printers, processes, metrology, etc. and, hence, is becoming economically unjustifiable. For a current account of overlay difficulties, see Yanof et al. "Overlay Measurement and Analysis of X-ray/Optical Lithography for Mix-and-Match Device Applications" SPIE Proc. 2194, pp. 36-50 (1994) page 50 in the "Conclusions", where it is stated that the overlay errors in optical to X-ray matching are as large as 60-70 nm from x-ray mask distortion alone, with distortion of optical mask and printer being comparable in magnitude (and additive in the practice of the prior art).
Edge-to-edge overlay (separation) of opening E with reference to opening B is critically dependent on centerline level-to level overlay (O/L) and on tolerances of linewidth (critical dimension, CD) for both openings B and E. In the prior art, a tight control of CD for both the openings B and E, as well as O/L of the opening E upon the opening B are individually maintained as is demanded by the market place, and accordingly, there is no alternative to that approach. The unacceptable alternatives are relaxed device ground rules which would result in non-competitive (slow, much energy consumption) or use of processes unable to support the required dimensional control and resulting in manufacture of unreliable devices.